Astronauts operate in a unique environment where gravity is almost absent: low Earth orbit. This weightlessness transforms their daily life, including simple actions like knowing their "weight". But how do they do it when a conventional scale is ineffective in space?
Astronaut Karen Nyberg uses the Space Linear Acceleration Mass Measurement Device (SLAMMD) aboard the space station.
Credit: NASA
On the International Space Station (ISS), two ingenious devices, developed by NASA and Russia, allow astronauts to measure their body mass. This distinction between weight and mass is crucial in microgravity, where only mass remains a reliable reference. Basic physics concepts are essential here to ensure their health.
Unlike weight, which is influenced by gravity, mass corresponds to the amount of matter composing a body. On Earth, these two concepts are often confused, but in space, where gravity is almost nonexistent, only mass can be measured using adapted methods. SLAMMD (Space Linear Acceleration Mass Measurement Device) relies on the principle of acceleration and force, formulated by Isaac Newton. A series of springs applies a known push, and an optical sensor measures the astronaut's response.
The process is both precise and simple: SLAMMD calculates acceleration based on speed and the time needed to cover a given distance. This data, combined with the applied force, allows body mass to be deduced with an accuracy of about 0.2 kilograms. This device is located in the European Columbus module of the ISS.
Russia uses another technique via the BMMD (Body Mass Measurement Device), located in the Zvezda module. This system, comparable to a pogo-stick, operates through oscillations caused by the astronaut. The frequency of the oscillations depends on the user's mass, allowing a quick and reliable measurement.
An astronaut using the BMMD, the Russian method for measuring body mass aboard the ISS.
Credit: NASA
Why this vigilance about body mass? In microgravity, astronauts experience rapid loss of bone density and muscle mass. Each month in orbit leads to a decrease of about 1% in bone mass. These changes require daily intensive training and regular monitoring to prevent long-term health consequences.
Physical exercise aboard the ISS lasts two hours per day to counteract these effects. Measuring body mass ensures precise tracking of routine effectiveness and guarantees that astronauts maintain optimal physical condition for their return to Earth.
What is the difference between weight and mass?
The distinction between weight and mass may seem subtle, but it is fundamental in physics. These two concepts, often confused in everyday language, differ in their definition and their relationship to gravity.
Mass is an intrinsic property. It measures the amount of matter in an object, expressed in kilograms or pounds. It remains constant whether the object is on Earth, the Moon, or in space.
Weight depends on gravity. It is a force, calculated by multiplying mass by the local gravitational acceleration. Thus, an object's weight varies according to the gravitational environment.
In microgravity, as in space, weight is nearly zero. However, mass remains unchanged. Astronauts therefore use specific devices to measure their mass, because traditional scales based on gravity do not work.